To develop clinical approaches that will improve the speed and quality of immune reconstitution after Hematopoietic Stem Cell (HSC) transplantation, it is necessary to understand in detail the biology of the primitive cells that engraft and generate short-term and long- term lymphoid production and the mechanisms by which this process is regulated. Many fundamental questions remain about this process in humans. The overall goals of this proposal are to identify and characterize primitive human lymphoid progenitors in clinically relevant sources of HSC and to determine how engraftment and differentiation of extrathymic and thymic human lymphoid progenitors can be manipulated after transplantation. We have recently identified and characterized a human Common Lymphoid Progenitor (CLP) population in umbilical cord blood and have identified a similar population in bone marrow. A number of critical differences exist between human and murine CLP, and between CLP from different hematopoietic sources emphasizing the importance of studying human cells from clinically relevant sources. Our Specific Aims are (1) To identify and characterize human CLP and other lymphoid progenitors in Mobilized Peripheral Blood (MPB); (2) To determine the thymic engraftment pattern of human HSC and lymphoid progenitors during homeostasis and after transplantation; and (3) To determine methods to enhance human T lymphopoiesis after HSC transplantation by in vivo and in vitro manipulation of the graft. A combination of in vitro and in vivo models will be used to accomplish these aims. Clonal assays will be used to prove the lineage potential of each population identified. The relative levels and sites of engraftment and the kinetics of engraftment of CLP and HSC from MPB, cord blood, bone marrow and thymus will be assessed. The role of Notch signaling in lymphoid commitment and expansion from human HSC and CLP will be determined.